The fringe of frozen sea surrounding Antarctica may have shrunk by as much as 25% between the mid-1950s and the early 1970s, according to a new study. The findings are based on a novel source: the logs of whaling ships. The study, reported in tomorrow's issue of Nature, suggests that the ice's retreat may be associated with global warming, but other researchers remain cautious.

If global temperatures are on the rise, some scientists say, one of the first things to go should be the vast regions of sea ice that surround Antarctica. But Antarctica's brutal climate, where temperatures plummet to -21 degrees Celsius in the dark winter months, has made firsthand observations difficult. Since the 1970s, satellites have watched the ice, which grows and shrinks by thousands of kilometers with the seasons. But data before that has been hard to come by. Hoping to peer further into the past, William K. de la Mare, a climatologist in Tasmania, Australia, dug through 60 years of whaling records.

Before whaling was banned by international treaties, whaling ships pursued blue whales to their preferred feeding grounds at the food-rich boundary between the Antarctic ice and sea. De la Mare therefore reasoned that whaling records, which document where catches occurred, should track the location of the sea-ice boundary. Working from over 40,000 records, de la Mare used the position of the southernmost kills to infer the location of the sea-ice edge at its maximum recession. Between the mid-1950s and early 1970s, he concluded, the summer edge beat a hasty retreat, sliding southward by 2.8 degrees of latitude, corresponding to a 25% reduction in coverage. He tested the technique by comparing his results from the 1970s to satellite data and found that the two methods agreed. The retreat, suggests de la Mare, is consistent with the notion that global warming would shrink the southern ice.

But other researchers hesitate to make that jump. Extracting scientific data from historical documents "is very difficult, painstaking work," says Claire Parkinson, a climatologist at the NASA Goddard Space Flight Center in Greenbelt, Maryland, who has used the journals of the 18th century explorer Captain Cook to reconstruct information about the sea ice of old. It might be, she suggests, that with declining whale populations, fishers simply had to push further south to make the kills. And, if de la Mare is right, she says, it doesn't mesh with temperature records, which show that temperatures in the southern latitudes were relatively stable during the years when de la Mare claims the ice retreated. "My best guess is that all these things we're seeing are natural fluctuations," she says. "Humans are certainly having an impact, but it's not established what that impact is."